Light Wiring Diagrams

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Ceiling rose

The ceiling rose, sometimes called a ‘loop-in ceiling rose’ or ‘3 plate ceiling rose’ is effectively a junction box for the power feed, switch wire and pendant flex of a ceiling light:

Ceiling rose with pendant attached

Ceiling rose with pendant attached

 

The ceiling rose has 3 sets of discreet terminals for the current carrying conductors plus an additional terminal for terminating the earthing conductors.

The diagram below shows the terminals in the ceiling rose plus typical termination of the cables:

Ceiling rose (new harmonised cable colours)

Ceiling rose (new harmonised cable colours)

 

 


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Multiway switching and switches – UK and US terminology

If you are searching online for 2 way switches/switching and 3 way stitches/switching the results can be very confusing to the layman depending on wether you land on a US or UK website. This is because the terminology used is very different.

This site follows UK usage. So for the benefit UK visitors searching online and American visitors to this site, hopefully the following will help to clarify:

Switches

2 way switch (UK) 3 way switch (US)

Fig 1: 2 way switch (UK) 3 way switch (US)


UK
– So, this (Fig 1) is what we call a 2 way switch: when you switch it one ‘way’ (on or position 1) the COM terminal is connected to the L1 terminal. When you switch it the other ‘way’ (off or position 2) the COM terminal connects to the L2 terminal.

If you are describing this switch to an electrician or electrical supplier you should refer to it as a 1 gang 2 way light switch and they will know exactly what you mean (see gangs and ways).

US – Strangely, our friends across the pond refer to this switch as a 3 way switch because it has 3 terminals.

Generic – if you asked someone in the field of electronics, they would probably describe this switch as a Single-pole, Double-Throw (SPDT) switch, to my mind a more useful description as this means the same in UK and US. A good electrician would understand this terminology but they, historically, rarely use it.

Intermediate or crossover switch (UK) 4 way switch (US)

Fig 2: Intermediate or crossover switch (UK) 4 way switch (US)


UK
– so, this (Fig 2) is what we call an intermediate switch, so called because it is used ‘in between’ two x 2 way switches to achieve 3 way switching (clarified below under multiway switching). Sometimes called a crossover switch for obvious reasons.

If you are describing this switch to an electrician or electrical supplier you should refer to it as a 1 gang intermediate light switch and they will know exactly what you mean.

US – in America they refer to this switch as a 4 way switch because it has 4 terminals.

GenericDouble-pole, Double Throw (DPDT) switch.

Multiway switching

Now we understand the different terminology used for the actual switches it is easy to appreciate why different switching arrangements are referred to using different terminology also.

Scenario 1: we have 1 light that can be turned on and off from two different light switches:-

UK – we would call this 2 way switching. Why? well a layman probably uses this terminology because 2 switches are used, whereas an electrician because two 2 way switches are used to achieve this. Either way, we are both referring to the same thing.

US – This would be referred to as 3 way switching because its uses what they would refer to a two 3 way switches.

Scenario 2: we have 1 light that can be turned on and off from three different light switches:-

UK – we would call this 3 way switching, well a layman certainly would (because of the three switches) and an electrician would assume this is what is meant but may also refer to it as 2 way plus intermediate switching.

US – This would be referred to as 4 way switching because it utilises what they refer to as a 4 way switch (what we in the UK call an intermediate or crossover).


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5 Amp 4 Terminal Junction Box

Juction box for lighting circuits

Juction box for lighting circuits

Here we have a 5 amp 4 terminal junction box that is ideal for use in lighting circuits. You will often see these used in old looped radial circuits instead of a ceiling rose in older houses. They are useful for ‘breaking in’ to an existing radial power loop to add additional wall lights in a room for example.

terminal block

terminal block

You can use our old friend the choc-block (nasty avoid it) but it will need to be in a suitable enclosure and kept away from anything that may give off heat (the plastic is much softer than the JB above) so you may as well use our friend above for lighting circuits.


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GANGS and their WAYS explained

No this is not a post about some dodgy Ross Kemp program:) Here we will attempt to clarify a subject that causes much confusion amongst  you budding D.I.Y. types…

Single gang 2 way, Triple gang 1 way, 2 gang 1 way, bla-d-bla-de-bla. So what the f**K does it all mean? Panic not my friends, it’s actually quite simple (although you wouldn’t think it if you have been reading many of the popular D.I.Y blogs).

Forget about the ‘WAYS’ first concentrate on the ‘GANGS’

So, just think of a gang as a single switch, simple as. If your light switch has one ‘switch button’ on it then it’s a single (or 1) gang switch. If it has two ‘switch buttons’ on it then it’s a double (or 2) gang switch. etc. etc.

single gang switch face

single gang switch

Double gang swith face plate

Double gang switch

Triple gang switch face

Triple gang switch

Glad we got that sorted?

‘GANGS’ are easy, so what of their ‘WAYS’

Well, as far as light switches (or gangs) are concerned, they can only be one of two things; a one way switch or a two way switch, end of.

One way light switch mechanism

One way light switch mechanism

A one way switch has two terminals, its the simplest of switch arrangements. it’s either on or off, thats all it does. When it’s ‘ON’ the COM terminal is connected to the L1 terminal (let there be light!). When it’s ‘OFF’ the COM terminal is connected to nothing and (the switch is open) no current flows through the switch.

 

 

 

Two way light switch mechanism

Two way light switch mechanism

A two way switch has three terminals its a little more complicated (any useful) than it’s one way cousin. When it’s ‘ON’ (position 1) the COM terminal is connected to the L1 terminal. But when it’s ‘OFF’ (position 2) the current is diverted from the L1 to the L2 terminal. This is what we use in circuits when we want to be able to switch a single light on and off from two different switches (see the two way switching lighting diagrams).

So, by way of a little revision

single gang switch face

single gang switch face

If we have a switch with one “switch button’ (1 gang) it can have a switch mechanism that is either one or two ‘WAYS’ so we have two possible options;

1/ A single GANG one WAY switch or

2/ A single gang two WAY switch

 

 

Double gang switch face plate

Double gang swich face plate

If we have a switch with two “switch buttons’ (2 gangs) it can have a switch mechanism that is either one or two ‘WAYS’ so we have two possible options;

1/ A Double GANG one WAY switch or

2/ A Double gang two WAY switch

And so on and so forth, Simples, you get the picture (I hope), I’m of to bed…

 


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Single gang 1 way light switch

There are two terminals in a one way light switch. When the switch is on, both terminals are connected together.

Typically, these terminals will be marked COM and L1 (sometimes L1 and L2). Although, technically, it doesn’t matter which way round you connect the wires, it is best to stick to convention and connect the permanemt live (from the supply) to COM and the switched live (to the lamp) to L1.

This is the most common type of switch, and is used where a light is controlled from a single switch (although you will often see a two way switch used, with one terminal left unused).

single gang switch face

single gang switch face

Single gang one way light switch

Single gang one way light switch

Switch mechanism

One way light switch mechanism

One way light switch mechanism

This is fairly obvious, but when the switch is ‘on’ the COM and L1 terminals are connected together. When ‘off’ they are not.


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Single gang 2 way light switch

A two way light switch has three terminals and is quite like a sigle way switch but now the ‘off’ position can be utilised also.

Some brands of switch may have slightly different labeling on the terminals (such as C L1 L2 or even L1 L2 and L3).

This type of switch is used where two switches control the same light, such as at the top and bottom of a stairway. You can use it as a one way switch, by using the COM and L1 terminals only.

single gang switch face

single gang switch face

Single gang two way light switch

Single gang two way light switch

Switch mechanism

Two way light switch mechanism

Two way light switch mechanism

In position 1 (when the switch is down or ‘on’), COM and L1 are connected together (just like the one way switch). In position 2 (when the switch is up or ‘off’), COM and L2 are connected together.

There is never a connection betweeen L1 and L2.


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New lighting circuit cable colours (harmonised)

If the wiring in your house is newer than the 31st March 2004  you will have the following ‘new cable core colours’ (or harmonised) used in your lighting circuits:

Twin and earth cable

This is the standard format that will be used for the vast majority of your lighting circuit.

New colour twin and earth lighting circuit cable

New colour twin and earth lighting circuit cable

3 core and earth cable

If you have two way switching (e.g. landing lights with a switch upstairs and down) or intermediate switching the cable format below may be used.

New colour 3 core and earth lighting circuit cable

New colour 3 core and earth lighting circuit cable

If the wiring in your house is older than the 31st March 2004 and you have had some work done (e.g. an extension or partial rewire) you may have a combination of the old  and the new ‘harmonised’ colours shown above.

On the 31st March 2004 The IEE published Amendments No2. to BS 7671:2001 (the IEE Wiring Regulations) specifying new cable core colours for all new fixed wiring in UK electrical installations. These colours are called ‘harmonised’ colours as they are closely related to those used in mainland Europe.

NOTE: the bare earth wire in the cables above should be sheathed with a green/yellow earth sheathing wherever it is exposed e.g. in light switches, ceiling roses, junction boxes, fand use boxes.


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Old lighting circuit cable colours (unharmonised)

If the wiring in your house is older than the 31st March 2004 (and hasn’t been modified) you will have the following ‘old cable core colours’ used in your lighting circuits:

Twin and earth cable

This is the standard format that will be used for the vast majority of your lighting circuit.

Old colour twin and earth lighting circuit cable

Old colour twin and earth lighting circuit cable

3 core and earth cable

If you have two way switching (e.g. landing lights with a switch upstairs and down) or intermediate switching the cable format below may be used.

Old colour 3 core and earth lighting circuit cable

Old colour 3 core and earth lighting circuit cable

This is another variation of an older 3 core and earth cable

3 core and earth 0ld cable colour variation

3 core and earth 0ld cable colour variation

If the wiring in your house is older than the 31st March 2004 and you have had some work done (e.g. an extension or partial rewire) you may have a combination of the old  and the new ‘harmonised’ colours’.

On the 31st March 2004 The IEE published Amendments No2. to BS 7671:2001 (the IEE Wiring Regulations) specifying new cable core colours for all new fixed wiring in UK electrical installations. These colours are called ‘harmonised’ colours as they are closely related to those used in mainland Europe.

NOTE: the bare earth wire in the cables above should be sheathed with a green/yellow earth sheathing wherever it is exposed e.g. in light switches, ceiling roses, junction boxes, fand use boxes.


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2 gang 1 way light switch

You don’t see these as much now days as many manufactures only make 2 gang 2 way light switches that serve the same purpose (you just don’t use the third L2 terminal on each gang), however I thought we would include it just in case you come across one!

Basically, this is just two single way switches on one face plate

Double gang swith face plate

Double gang switch face plate

Double gang one way light switch

Fig 2: Double gang one way light switch

Switch mechanism

One way light switch mechanism

One way light switch mechanism

Each of the gangs (or switches) above in Fig 2 (of which there are two) work like this: when the switch is ‘on’ the COM and L1 terminals are connected together. When ‘off’ the COM is connected to nothing.

There are no connections between the two gangs in Fig 2. Each gang is a single discreet switch.


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Twin and earth cable current ratings

When considering cable ratings care needs to taken as this is not as straight forward as it may seem. For example, if you check the manufacturers current rating for 1.5 twin and earth cable (suitable for  most lighting circuits in a domestic environment) it will state that it is around 20 amps, but this value would only apply if the cable was in free air (as it can cool easily). If you put the same cable above a plasterboard ceiling covered by thermal insulation 100mm thick (17th edition regs reference method 100) its current carrying capacity is reduced to 16 amps (because it can no long cool as easily).

Consider the table below and the reference methods below that to see how the current carrying capacity is affected by installing the cable in different situations.

Conductor area (mm²)

Current carrying capacity (AMPS)
for various reference methods
(see below)

A

B

C

100

101

102

103

1

11.5

13

16

13

10.5

13

8

1.5

14.5

16.5

20

16

13

16

10

2.5

20

23

27

21

17

21

13.5

4

26

30

37

27

22

27

17.5

6

32

38

47

34

27

35

23.5

10

44

52

64

45

36

47

32

16

57

69

85

57

46

63

42.5

Examples of installation method

BS7671 17th Editon
Reference Method
Example of installation method
A In conduit in an insulated wall with the conduit close to or touching the inner skin.
B Enclosed in conduit or trunking on or in a wall.
C Clipped direct, or sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)
100 Above a plasterboard ceiling covered by thermal insulation, insulation thickness less than 100 mm.
101 Above a plasterboard ceiling covered by thermal insulation, insulation thickness greater than 100 mm.
102 In a stud wall with thermal insulation with the cable touching the inner wall surface.
103 In a stud wall with thermal insulation with the cable NOT touching the inner wall surface.

Cable sizing example

So lets say we wish to rewire a lighting circuit for the upstairs of our house. We have 3 bedrooms, a bathroom and a landing light. So lets assume we will be powering 5 100w lamps which gives us 500 watts in total.

We plan to use our standard 1.5mm twin and earth cable that the manufacturer rates at 20 amps, but the cables are run under thick (more than 100mm) insulation in the loft (installation method 101) so our cables current carrying capacity is reduced to 13 amps (from top table).

So if we now take our total wattage and divide it by the voltage of the circuit we will see how much current (amps) our lights will draw:

500(watts) / 240(Volts) = 2.08 amps

This is well within the 13 amp current carrying capacity of our cable so the 1.5 twin and earth cable will be fine for the job.

Note

This is quite a crude calculation, circuit designers will take additional factors into consideration when sizing cables, for example, in large installations the length of cable (and the resulting volt drop along it, due to its resistance) will need to be considered. Also my power calculation is not entirely accurate because we would need to consider the power factor of the circuit; inductive loads (like the chokes in fluorescent lamps and low voltage lighting transformers) have the effect of putting the current and the voltage in the circuit ‘out of phase’ which will affect the current drawn.

In summary, if you have an average size 3 -4 bedroom house with run-of-the-mill lamps and the odd fluorescent lamp, and after doing the calculation above you are well within the current carrying capacity of the cable (after allowing for the installation method as described) and you protect the circuit with a 6 amp circuit breaker you can’t go far wrong.

In a large dwelling (with potentially long cable runs) and a mix of exotic lighting with low voltage lighting transformers etc  the basic calculations above may not be sufficient and I would recommend consulting an expert.


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